Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180324

Area Forecast Discussion
National Weather Service Sioux Falls SD
924 PM CST Sat Nov 17 2018

.SHORT TERM...(This evening through Sunday)
Issued at 347 PM CST Sat Nov 17 2018

Current surface analysis shows a cold, 1036mb high centered near
Aberdeen and Huron SD. There are a couple of items to deal with
tonight. In the near term, stratus lingers over much of northwest
IA. Satellite is showing this stratus as cold air advection
streamers, therefore it should dissipate near sunset as it advects
slowly southward. Of bigger concern is the widespread stratus in
central and western SD. The satellite is showing this stratus to be
rather thin, which is possible the reason why every model I can find
has no clue that it exists. As the current surface high moves
southward, the low level winds will become southwesterly as the
night progresses. Therefore one would think that the stratus will
ooze into our western zones, and possibly some of our northern
zones. Therefore manually drew some cloud cover in for these areas
and the evening shift will have to reevaluate this after several
hours. The stratus will likely setup in a south southwest to north
northeast orientation, aided by a strengthening inversion over the
snow cover in northern SD after sunset. So it is really a question
mark how far east these clouds will get in the FSD CWA.

The west river stratus will definitely not move east enough to keep
lows from plummeting over our northeast zones where snow cover
resides however. Dropped temperatures near Brookings and southwest
MN to readings much below guidance values this evening, with lows
near zero. In fact they are not too far from there right now. But
overnight once the southwesterly flow kicks in, even our snow
covered areas will begin to steady out with slowly rising
temperatures after midnight. This will lead into a much milder
Sunday with the southwest surface flow continuing. With weak low
pressure moving into western and central SD, a warm front moves
eastward and extends across our southwest zones. Therefore went
above consensus highs in our Missouri River valley zones with
readings in the lower to mid 40s. Conversely, kept highs a bit below
consensus highs in our northeast zones, with highs near 30 or lower
30s in southwest MN.

.LONG TERM...(Sunday night through Saturday)
Issued at 347 PM CST Sat Nov 17 2018

The aforementioned western plains low pressure system moves eastward
Sunday night with a cold front dragging southward behind it pushed
by high pressure in southern Canada. Increased winds behind this
front by using consensus mos values late Sunday night and Monday. In
addition, cooled highs below superblend on Monday by blending in
some bias corrected ECMWF. Stratus in the 925-850mb layer will
likely move southward with the cold air advection late Sunday night
and Monday, lingering in our far eastern zones through Monday
afternoon. To the west, a classic baroclinic zone sets up in the
same layer with a tight thermal gradient across central SD.
Therefore another area stratus is likely west of the James River
valley on Monday. Did not mention flurries anywhere with the stratus
at this time, as the stratus layer appears a bit too warm to support
ice if the current model 925-850mb thermal fields do not cool off
anymore in future runs.

On Monday night, the aforementioned stratus in our western zones
will move eastward in a band associated with a warm front, as the
high moves off to the southeast. Therefore the winds will become
southwesterly again, with a seasonably pleasant day on Tuesday under
a southwest flow. For Wednesday, similar to yesterday the ECMWF
continues to affect our forecast area more vigorously with a cold
front moving southward through the Great Lakes region as opposed to
the GFS. For example at 00Z Thursday, the ECMWF 925mb temperatures
range from -5C in southwest MN, to +3C in south central SD.
Conversely, the GFS 925mb temperatures range from +3C in southwest
MN, to +10C in south central SD. Therefore we are talking about 7 to
8 degrees C difference between the two model solutions, or about 14
degrees F in Wednesday highs. With this much discrepancy, just went
with superblend and we can reevaluate this tonight and Sunday.
Thanksgiving Day looks mild however albeit maybe breezy with winds
from the south.

Late in the week, the various models continue to show some upper
troughing moving into the plains, with perhaps a chance for
primarily light rain over parts of northwest IA on Friday.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 924 PM CST Sat Nov 17 2018

VFR through the period. Still a small chance for some MVFR
ceilings in central SD before 12z.




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